Studies on the design, preparation, characterization, and enzymology are proposed for the sphingolipid analogues (SPIs) of the phosphoinositides. The parent structure incorporates sphingosine (SN) esterified to the 1-phosphate of an inositol-1-phosphate, and is formally derived by replacement of the 1,2-diacyl-sn-glycerol residue in phosphatidylinositol (PI) by a SN moiety. The SPIs are putative substrate analogues for the phosphatidylinositol-specific phospholipases (PI-PLC), and are expected to provide rationally designed and practical probes for modulating, particularly attenuating intracellular signal transduction via the phosphoinositide and sphingolipid cascades. SPIs would be valuable in research on the mechanism of signal transduction, and may well form the basis of novel targeted prophylactic and therapeutic agents for physiological disorders associated with aberrant signal transduction especially some types of cancers. The Phase I objective is to establish if SPIs function as substrate or inhibitor in hydrolysis catalyzed by PI-PLC. For this, prototypical SPI analogues will be prepared by synthesis and their behavior in hydrolysis catalyzed by a bacterial PI-PLC studied in comparison with product profile of PI-PLC hydrolysis of PI. The SPIs offer opportunities for rational changes in the sphingosyl, phosphate and inositol residues for selective modification of the chemical, physical, topological, and membrane affinity characteristics, and for incorporating antigenic glycan residues, designed to achieve the optimum substrate-mimetic performance, and receptor-targeted delivery. These opportunities, studies on the behavior towards selected mammalian PI-PLCs, and other enzymes pertinent to the signalling cascades, and their natural occurrence as mediators in cell physiology, will be investigated during Phase II.